Method of producing phosphates



J. W. EDWARDS METHOD OF PRODUCING PHOSPHATES Filed Feb. 14, 1957 Jan. 12, 1960 0 O O 8 w 4 #55:: z h o l z kzwo Em 5 INVENTOR.

JAMES W. EDWARDS Bid/ f HIS AGENT TIME, IN MINUTES United States Patent 2,920,939 METHOD OF PRODUCING PHOSPHATES James W. Edwards, Dayton, Ohio, assignor to'Monsanto Chemical Company, St. Louis, Mo., a corporation of Delaware Application February 14, 1957, Serial No. 640,256

6 Claims. (Cl. 23-106) This invention relates to novel improvements in methods for producing alkali metal tripolyphosphates, such as the sodium, potassium, lithium, etc., tripolyphosphates.

Most of the tripolyphosphates now produced commercially are manufactured by a so-called solid phase process which involves heating a mixture of phosphates other than tripolyphosphates (e.g., a mixture of rnonosodium orthophosphate and disodium orthophosphate) at a temperature below the melting point of the tripolyphosphate until the mixture has been converted to the desired tripolyphosphate. The time required for conversion to tripolyphosphate in such a process will vary somewhat depending upon such factors as temperature, particular phosphate reactants selected, etc. It has now been found that these processes can be carried out much more effectively if an appreciable amount of crystalline tripolyphosphate is initially introduced into the reaction mixture.

The phosphate mixtures which are useful for conversion according to the present process are those in which the overall molecular ratio of alkali metal oxide to phosphorus pentoxide is between about/1.5 and about "1.8, and preferably between about 1.6 and about 1.7, and in which the overall molecular proportion of alkali rnetal oxide to water of constitution (as distinguished from Waterof hydration) is less than about IQ, and preferably less than about 5. .The phosphate mixtures can be made up of ortho., pyro-,, meta-, or intermediate glassy phosphates, or combinations thereof, can beeit'her hydrated or anhydrous, and may include .normal'salts as Well acid salts. While it' is generally more convenient to use a mixture of only two different phosphate salts, mixtures of more thantwo can 'belutilized equally well. 1

Examples of suitable phosphate mixtures for conversion to tripolyphosphates according to the present invention are as follows:

(l) 65 weight percent trisodium hydrogen pyrophosphate monohydrate and 35 weight percent disodium orthophosphate.

(2) 65 weight percent monosodium orthophosphate-disodium orthophosphate double salt and 35 weightpercent disodium orthophosphate. V r

(3) 60 weight percent monosodium orthophosphate-disodium orthophosphate double salt and 40 weight -percent disodium orthophosphate dihydrate. 1

(4) 60 Weight percent trisodium hydrogen pyrophosphate monohydrate and 40, Weight percent gdisodium orthophosphate dihydrate. I

('5) 22 weight percent monosodium orthophosphate and .78 weight percent disodium'orthophosphate.,

(6) 25 weight percent monosodium orthophosphate and "75 weight percent disodium orthophosphate dihydrate.

(7) 20 weight percent monosodiurn orthoph'osphate and 1 80 weight percent tetrasodium pyrophosphate decahydrate. H

. 2,920,939 ,Fatentecl Jan. 12, 196.0

(8) 39 percent .rnonosodium orthophosphate and 61 weight percent trisodium orthophosphate dodecahydrate.

(9) '9 weight percent sodium acid pyrophosphate, 85

weight percent tetrasodium pyrophosphate decahydrate and 6 weight percent monosodium orthophosphate.

(1Q) 15 weight percent sodium metaphosphate, 18 Weight percent .monosodium orthophosphate and 6'] weight percent disodium orthophosphate dihydrate.

( 11) 28 weight percent monopotassium orthophosphate and 72 Weight percent dipotassium orthophosphate.

(12) 5'6 weightpercent monopotassium orthophosphate and 44 weight percent tr-ipotassium orthophosphate.

(13) The double salt hydrate, KH PO .2K HPO .H O.

(14) 44 weight percent dipotassium orthophosphate trihydrate and 56 weight percent tripotassium monohydrogen pyrophosphate.

(l5) 64 weight percent monolithium orthophosphate-and 36 weight percent trilithium orthophosphate.

In order to produce t-ripolyphosphates having a mini? mum of other phosphate impurities therein, the propor tion of the particular phosphate reactants chosen should be adjusted so that the alkali metal oxides and-phosphorus ,pentoxide present in the mixture have a mole ratio of 5 :3 (the ratio of oxides in pure tripolyphosphate). The use of mixtures havinga mole ratio of alkali metal oxides to phosphor-us pentoxide other than 5: 3 will result in products containing non-tripolyphospha'teimpurities, the proportion of impurities increasing as the oxide'ratio of the reaction mixture departs ffarther from 5:3. As indicated before, "however, the pres ent invention is applicable not only to phosphate mixtures containing a mole ratio of oxides of 5:3, but also to phosphate mixtures in which the ratio varies somearr m.

The discovery upon which the present invention is predicated is that the conversion of the above-described phosphate mixtures to tripolyphosphategor to mixtures containing predominantly tripolyphosphates, can be carried out much more eiiectively and economically if some crystalline tripolyphosphate is initially present in the reaction mixture. .The exact amount of crystalline tripolyphosphate to be used will ;be largely dependentupon the economics. applicable to the particular process; but in general greater than 1 weightpercent, and preferably greater than about 2 weight percent will be most feasible. There is no upper limit to the amounts that can be used, but theeifectiveness per unit weight of added tripolyphosphate decreases somewhat with increasing amounts added, so that it will often be uneconomical to use undulylarge quantifies. On the other hand, there will be some occasions when it will be both economical and necessary to utilize unusually large proportions ofcrystall-ine tripolyphosphate in the initial phosphate reaction mixtures-such as in the production of very pure products. For example, the so -called,low-temperature form of sodium tripolyphosphate (Na P O --II, discussed in rea detai h r ina n e produced pr ctica ly completely free from impurities (such as insoluble metaphosphates) by making up reaction mixtures containing several times (e.g.', as much as -8 or 10 times). as much crystalline Na P O II as non-tripolyphosphate reactants. I

The times .and temperatures required to convert the various phosphate reaction mixtures described above to tripolyphosphates according to the present invention will depend (somewhat upon the particular tripolyphosphate to be produced and the particular reactantsutilized, as well as upon the proportions and physical -l-state's (-i-.e.,

reactants. In general, the temperatures utilized will be somewhat above 175 C., and preferably above about 200 C.

As already indicated above, the advantages of the present invention may be manifested in one or more of several ways, such as by greater yields of desired product, accelerated rates of reaction, greater degrees of freedom in selection of reaction conditions, etc.

While the present invention is generally applicable to any of the alkali metal tripolyphosphates, it is particularly advantageously utilized in the production of sodium tripolyphosphatesand especially in the production of the so-called low-temperature sodium tripolyphosphate, Na P O -II. (Sodium tripolyphosphate, Na P O is known to be a polymorphic material, of which at least two separate and distinct solid crystalline modifications have been identified. These modifications are commonly referred to as Form I and Form II, and are designated by the formulae, Na P O -I and Na P O II, respectively. See Partridge et al., Journal of the American Chemical Society, vol. 63, page 454 et seq.). Na P O II is generally formed at temperatures below about 470 C., and preferably below about 400 C., whereas Na P O -I is generally formed at temperatures inexcess of about 500? C. Probably because of the lower conversion temperatures utilized, the conversion to Na P O II is somewhat slower than the conversion to Na P O -I, and is also more prone to production of a product having noticeably higher contamination with insoluble impuritiessuch as insoluble metaphosphates. By using the improved process of the present invention, it is possible to effectmarked improvements in both the conversion rate and product purity of Na P O II. Although any of the crystalline forms of sodium tripolyphosphate (for example, Na P O I, Na P O -lLor Na P O -6H O) can be used to improve the production Df N35P3010II, the Na P O 6-I and N35P3010'6H20 are found to be somewhat more effective than N215P3O10II. I I

The following examples are presented as illustrative of various preferred embodiments of the present invention, and as indications of the markedly improved results which can be obtained thereby.

Example I One-tenth gram of powdered Na P O II and 1.9 grams of an equimolar mixture of Na HP,O and NaH P -Na HPO were thoroughly mixed together and heated in an open c'rucible for two hours in a muflle furnace at 200 C. At the same time, 2.0 grams of the equimolar mixture of Na HPOpwith NaI-I PO -Na HPO (but with no-Na P O added) was heated in the same furnace under the same conditions. Subsequent analyses of the reaction products showed that the reaction mass to which five percent of Na P Q II had been added, had been converted to about 45 percent by weight of Na P O whereas the control sample (no Na P O added to reactants) contained only 22 weight percent of S S IO- T i 7 Example 2 Three-tenths gram of powdered K -P O and 2.7 grams of a mixture containing two molesof K HPO with one mole of KH lO were thoroughly mixed together and heated in an open crucible for 30'minutes in a muffle furnace at 300? At the same time, 3.0 grams of the 2:1 K HPO -KH PO mixture was heated in the same furnace under the same conditions. Subsequent analysis of the reaction products showed that the mixture to which K P O had been added had been converted to about 87% by weight of K P 0 whereas the control sample (no K PgO added to the reactants) had been converted to only 66% weight percent K P O I 4 Example 3 Six-tenths grams of Na P O I, was ground to smaller than ZOO-mesh and intimately admixed with 11.4 grams of unground flakes of equimolar proportions of NaH PO 'Na HPO and Na HPO '2H O. This mixture was then divided into 6 equal proportions of 2 grams each and placed in an oven at 250 C. Each sample was left in the oven for a different length time, then withdrawn and analyzed for Na P O II. In this way it was possible to determine the rate of conversion of the phosphate mixtures to sodium tripolyphosphate-II.

The foregoing procedure was repeated except that 0.6 grams of Na P O -6H O was used in place of the 0.6 gram of Na P O -I. The procedure Was also repeated using just the flakes of NaH PO -Na HPO and Na HPO -2H O, without any additional materials.

The results of the foregoing are plotted as the curves of the drawing, and illustrate the marked effect which as little as 5 percent of Na P O I or Na P O -6H O has upon the conversion of phosphate mixtures to Na5P3O o-II.

In the preceding examples the crystalline tripolyphosphate additive was introduced as a solid directly into the solid phosphate mixture which was to be converted to tripolyphosphate product. However, the tripolyphosphate additive can be introduced at earlier stages of the processsuch as during the prepartion of the phosphate mixture which is to be converted. For example, the phosphate mixture utilized in Example 1 can be prepared by evaporating the water from a solution of two mole proportions of disodium orthophosphate and one mole proportion of monosodium'orthophosphate. If the desired amount of crystalline sodium tripolyphosphate additive is dissolved inthis solution prior to evaporation, the solids resulting from the evaporation will contain the necessary crystalline tripolyphosphate for carrying out the process of the present invention.

This application is a continuation-in-part of my earlier copending applications Serial No. 398,725, filed December 17, 1953, now abandoned, and Serial No. 487,221, filed February 9, 1955, now abandoned.

I claim:

1. In the method of producing alkali metal tripolyphosphates by condensation of a mixture of phosphates other than tripolyphosphates at a temperature below the melting point of said tripolyphosphate, said phosphate mixture having an overall molecular ratio of alkali metal oxide to phosphorus pentoxidebetween about 1.5 and about 1.8 and an overall molecular ratio of alkali metal oxide to water of constitution less than about 10, the improvement which comprises adding at least 1 percent by weight of crystalline alkali metal tripolyphosphate to said mixture of phosphates, thereby accelerating the rate of conversion of said mixture of phosphates to said alkali metal tripolyphosphate.

2. The method of claim 1, wherein the alkali metal tripolyphosphate is sodium tripolyphosphate.

V 3. The method of claim 1, wherein the alkali metal tripolyphosphate is potassium tripolyphosphate.

4. In the method of producing sodium tripolyphosphate by condensation of an approximately 2:1 molar mixture 'of disodium orthophosphate and monosodium orthophosphate, said mixture of orthophosphates having an over-all molecular ratio of sodium oxide to phosphorus pentoxide between about 1.5 and about 1.8 and an overall molecular ratio of sodium oxide to water of constitution less than about 10, said condensation being carried outat a temperature below the melting point of said tripolyphosphate,'the improvement'which comprises adding at least 1 percent by weight of crystalline sodium tripolyphosphate to said mixture of orthophosphates, thereby accelerating the rate of conversion of said mixture of orthophosphates tosaid sodium tripolyphosphate.

5. The process of claim 4, wherein the amount of crystalline sodium tripolyphosphate added to said mixture of orthophosphates is at least about 2 percent by weight.

6. The method of producing sodium tripolyphosphate- II, which comprises adding at least 1 percent by weight ofa compound selected from the group consisting of sodium tripolyphosphate hexahydrate and sodium tripolyphosphate-I to a mixture of sodium phosphates other than sodium tripolyphosphate containing at least 10 percent by weight of orthophosphate and having an overall molecular ratio of Na O:P O between about 1.5 and about 1.8 and an overall molecular ratio of Na Ozwater of consti- References Cited in the file of this patent UNITED STATES PATENTS 2,419,148 King Apr. 15, 1947 2,493,809 Garrison Jan. 10, 1950 2,737,443 Wright Mar. 6, 1956 2,776,187 Pfengle Jan. 1, 1957 

1. IN THE METHOD OF PRODUCING ALKALI METAL TRIPOLYPHOSPHATES BY CONDENSATION OF A MIXTURE OF PHOSPHATES OTHER THAN TRIPOLYPHOSPHATES AT A TEMPERATURE BELOW THE MELTING POINT OF SAID TRIPOLYPHOSPHATE, SAID PHOSPHATE MIXTURE HAVING AN OVERALL MOLECULAR RATIO OF ALKALI METAL OXIDE TO PHOSPHORUS PENTOXIDE BETWEEN ABOUT 1.5 AND ABOUT 1.8 AND AN OVERALL MOLECULAR RATIO OF ALKALI METAL OXIDE TO WATER OF CONSTITUTION LESS THAN ABOUT 10, THE IMPROVEMENT WHICH COMPRISES ADDING AT LEAST 1 PERCENT BY WEIGHT OF CRYSTALLINE ALKALI METAL TRIPOLYPHOSPHATE TO SAID MIXTURE OF PHOSPHATE, THEREBY ACCELERATING THE RATE OF CONVERSION OF SAID MIXTURE OF PHOSPHATES TO SAID ALKALI METAL TRIPOLYPHOSPHATE. 